Greenpeace activists display a banner saying "Is this banner going to disappear under the water in 50 years" on a bridge in Warsaw November 17, 2007. The activists were protesting against global warming. REUTERS/Greenpeace/Handout (POLAND). NO ARCHIVES. NO SALES.
Ran on: 11-18-2007
Members of Greenpeace display a banner that reads: &quo;Is this banner going to disappear under the water in 50 years?&quo; on a bridge in Warsaw. The group was protesting global warming.
Ran on: 11-18-2007
Members of Greenpeace display a banner that reads: &quo;Is this banner going to disappear under the water in 50 years?&quo; on a bridge in Warsaw. The group was protesting global warming.

If the nations of the world continue to pump tons of carbon dioxide into the atmosphere from smokestacks and tailpipes, the world will suffer catastrophic droughts and heat waves and rising seas that could kill millions of people, according to the most exhaustive work ever done on planetary climate change.

The arid and semi-arid lands, including the western United States, would be hit by runaway wildfires, have less drinking water from shrinking snowpacks, and face unpredictable ocean conditions that would upset the feeding cycles of krill, fish and other sea life.

But if the United States and China, the two top greenhouse gas producers, work with other countries to improve energy efficiency and invest in alternative technologies, the worst effects of global warming could be forestalled on Earth, according to a Nobel Peace Prize-winning U.N. science panel that released its final report Saturday.

The Intergovernmental Panel on Climate Change, a group of 2,000 scientists from 140 countries, has over the past several months been releasing volumes of its work on global warming. The report released Saturday is the synthesis of that work, a six-year endeavor to measure most of what is known about global warming and its effects on human health, the oceans, wildlife and how the world might adapt, among other topics. The first of the five-year assessments came out in 1990 and was used as the basis for the U.N. Framework Convention on Climate Change.

The summary report released Saturday is geared toward policymakers who will meet in Bali, Indonesia, next month to begin forging a treaty to replace the Kyoto Protocol, which expires in 2012.

Since 1970, global greenhouse gas emissions have grown by 70 percent, and are causing global warming, according to the report.

Maintaining today's rate of emissions from power plants, factories, vehicles and other sources could raise global temperatures 10 degrees by the end of the century. That would mean the extinction of more than half of the world's plant and animal species, creation of super-intensive hurricanes and melting of glaciers, according to the panel.

To limit the increase in global temperatures to 4 degrees - which would limit sea level rise to between 15 and 55 inches over the century - the world needs to cut emissions by 50 to 85 percent by 2050, the panel said. That reduction is close to what California law requires.

Over the past century, global temperatures rose 1 to 2 degrees, and the global average for sea level rise was 6 inches.

Some of the effects of global warming, such as melting land and sea ice, intense hurricanes and an increase in wildfires, are happening now and can occur with greater intensity as temperatures rise even a few degrees, the scientists said.

"The warming in the last 40 to 50 years is very likely due to human activities, mostly from our smokestacks and tailpipe waste and deforestation activities, which have increased carbon dioxide by 35 percent" since the Industrial Revolution, he said. "Although some risks are already unavoidable, there is still time to reduce the likelihood of really catastrophic impacts like melting ice sheets and massive species extinction."

Possible solutions include enlisting private industry to develop alternative technologies that are less emitting, according to the report. Government officials also could use tax incentives and subsidies, levy a tax on companies or people who emit greenhouse gases, or develop market-based systems in which people pay extra for the right to emit more greenhouse gases than otherwise allowed, the summary report said.

Despite the exhaustive work by the U.N. panel, much is still not known, the scientists said.

For example, it is not clear whether the planet is more likely to be 2 to 3 degrees or 10 degrees warmer by the end of the century. Other unknowns are the amount of sea level rise and how precipitation will be affected in different parts of the world.

The Carnegie Institution at Stanford University compared cities' annual average temperatures to get a feel for what a temperature increase would mean: Tucson is an average 10 degrees hotter than San Francisco, and Santa Barbara is, on average, 2.2 degrees warmer than San Francisco.

"We're going to continue to see sea levels go up, to see a continued dwindling of mountain snowpack in the spring when we need it most, and a drying out of the semi-arid West. Water resources are likely to become more limited," Overpeck said.

The summary emphasizes the uncertainty over sea level rise, he said.

"It's clear that scientists couldn't define the upper end of sea level rise by the end of the century. The ice sheets in Greenland and Antarctica are changing very rapidly in ways we still don't understand," Overpeck said.

Scientists expect a better estimate within the next five years. "An increasing number of scientists are saying we could have a meter or more by the end of the century" if the melting of glaciers in Greenland and other areas is considered, he said.

Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder and a coordinating lead author for part of the summary report, said the scientists also look for improvements in the way that they can predict ocean conditions or precipitation.

Refining and improving predictions is crucial, he said, in helping policymakers make decisions regarding fisheries, forests, water and agriculture.

Trenberth wasn't optimistic that policymakers would take the cautionary advice of the scientists and make deep cuts in emissions.

The United States leads in per capita emissions but has not actively participated in scientific assessments to pull together all that is known about global warming. In some cases, the U.S. government has at times sought to block such science, Trenberth said.

"The U.S. needs to play a substantial leadership role in Bali, and I just don't see that happening with this administration. If they do, I think other countries will be more inclined to follow. China and India are not going to lead in this," Trenberth said.

"The U.S. has the responsibility to lead but has not been doing so in recent years, in my view."

Online resources

Read the report:

Key findings

-- Global warming is unequivocal. Temperatures have risen 1.3 degrees in the last 100 years. Eleven of the past 12 years are among the warmest since 1850. Sea levels have gone up by an average 0.07 of an inch per year since 1961.

-- About 20 to 30 percent of all plant and animal species face the risk of extinction if temperatures increase by 2.7 degrees . If the thermometer rises by 6.3 degrees, between 40 and 70 percent of species could disappear.

-- Human activity is largely responsible for warming. Global emissions of greenhouse gases grew 70 percent from 1970 to 2004. The concentration of carbon dioxide in the atmosphere is much higher than the natural range over the past 650,000 years.

-- Climate change will affect poor countries most, but will be felt everywhere. By 2020, 75 million to 250 million people in Africa will suffer water shortages, residents of Asia's large cities will be at great risk of river and coastal flooding, Europeans can expect extensive species loss, and North Americans will experience longer and hotter heat waves and greater competition for water.